ATM technology has been designed with the intention of providing service for a wide variety of applications such as voice, video, and data. Each of these applications has different service requirements in terms of cell loss, delay, and cell delay variation (CDV). For example, voice traffic can withstand a small amount of cell loss, but it is rather intolerant to delay and CDV. Due to the asynchronous statistical nature of ATM, voice traffic must be smoothed at the receiver end in order to eliminate any accumulated CDV incurred in the network. The requirements for video traffic are somewhat dependent upon coding and compression schemes. These coding schemes can produce either constant bit-rate (CBR) or variable bit-rate (VBR) traffic. Video traffic can typically tolerate a small amount of cell loss, however, it is sensitive to delay and CDV (although generally less sensitive than voice). Data services must not lose any information. Cell loss should be very small in order to reduce retransmissions. Data can withstand a considerable amount of delay and CDV.
STM service is ideal for voice and CBR video traffic since CDV and cell loss due to congestion are eliminated. Of course, STM service wastes bandwidth when a particular connection sends no traffic, and VBR services (data or VBR video) cannot be efficiently serviced by STM techniques. It is therefore desirable to design a telecommunication switch which can handle both STM and ATM data.
There have been some proposals dealing with this objective. For example, U.S. Pat. No. 5,119,370 (Terry), issued Jun. 2, 1992, and U.S. Pat. No. 5,144,619 (Munter), issued Sep. 1, 1993, describe a joint STM and ATM network. These previously proposed STM/ATM networks are formed by separate STM and ATM switching equipment. Thus, each switching node is composed of an STM switch and an ATM switch. U.S. Pat. No. 4,926,416 (Welk), issued May 15, 1990, also teaches a similar arrangement in which STM and ATM switches are used for handling mixed traffic. Japanese Opened Application No. 2-224547, published Sep. 6, 1990, and Application No.1-148000, published Jun. 9, 1989, describe ATM/STM hybrid switching systems.
The invention described herein eliminates the STM switch by providing a scheduling mechanism in the ATM switch which provides both ATM and STM service. The present invention therefore relates to the scheduling of ATM cells sent on a transmission medium, and is particularly concerned with effectively providing a suitable quality-of-service for a wide range of traffic applications. Specifically, it is directed to a scheduling mechanism which supports both ATM statistical multiplexing and STM deterministic service on an individual connection basis within an ATM switch.